Very small aperture terminal (VSAT) networks have been developed for reliable and economical data transmission applications. The networks are implemented in satellite telecommunications "star" configurations, wherein a large number of remote VSAT stations access a main "hub" station. The hub is linked to a host computer or ground-based computer network. Remote VSAT stations can access the hub very quickly by relaying their transmissions via fixed-orbit satellites. VSAT technology has also been deployed in point-to-point transmission and receiving configurations known as "mesh" networks, which also operate via satellite relaying. Both of these VSAT network configurations are presently used in wide-ranging applications such as hotel reservation systems, corporate private data and voice networks, stockbroker activities systems, and video receive-only systems.
Due to the increased demand for voice and data service access through these networks, VSAT technology has been extended to telephony applications. "Telephony" systems carry voice and voice-band data from devices such as computers and facsimile systems. Telephony VSAT networks allow both voice and data services to be transmitted via satellite over vast distances almost instantaneously. The telephony VSAT networks are most often used to extend the public phone network into remote areas. Similarly, they are also used to provide emergency communications and disaster recovery. Telephony VSAT networks can also implement private voice/data systems by connecting a number of private branch exchanges (PBX).
A telephony VSAT system consists of multiple remote sites and a central site. Each site consists of an outdoor unit and an indoor unit. The outdoor unit comprises of an antenna, RF electronics and an interfacility link cable (IFL). The indoor unit preferably includes a distribution chassis and channel units. A channel unit provides the compatible interfaces and signalling to the public switched telephone network. The central site is used for network management and assigning calls between the different remote sites. When a call is established, the audio signal, voice-band data, or facsimile signal is modulated after baseband signal processing. The modulated signal is converted into an intermediate frequency (IF) signal and transmitted in time-partitioned packets called bursts. The IF signal is then up-converted to C-band or Ku-band signals and transmitted to a fixed orbit satellite.
The satellite relays the burst to a receiving antenna. The receiving RF electronics performs frequency translation to down-convert the incoming signal to IF. The down-converted signal is then fed into the channel unit. The channel unit has a demodulator which detects the incoming burst, estimates its parameters, and demodulates the burst. The demodulated data is then provided to the receiver baseband processor which converts it into a usable audio, voice-band data, or facsimile signal.
Telephony transmissions may be divided into two broad categories which utilize the same transmission network: 1) speech, which consists of short transmission bursts, typically less than a few seconds long; and 2) voice band data modems, fax, and other types of telephony traffic that have transmission bursts which typically last longer than one minute. Each of these types of transmissions has different characteristics within the bandwidth which must be addressed during demodulation at the receiving end.
While the second category of transmissions usually originates from a steady stream of data, speech transmissions almost always originate from conversant language. Human speech, by its very nature, is very choppy. Relatively long periods of silence are prevalent. In order to conserve power and optimize shared bandwidth during speech-based transmissions, voice activity detection (VAD) algorithms are often used to activate the modulation process only when speech is actually occurring. When VAD is enabled, the resulting modulated transmission is therefore choppy as well.
Bursts are made up of a preamble, user data, and a postamble. The preamble marks the beginning of the burst and the user data, and consists of a bit-timing recovery sequence and a "preamble unique word." The user data contains the voice and/or data information intended to be transmitted. The postamble is appended to the end portion of the transmission burst to mark the end of the user data. In order to determine the end of the user data, a "postamble unique word" is present within the postamble. When the demodulator detects the postamble unique word that matches a predetermined unique word, the demodulator resets and begins to seek the next burst. If the postamble unique word is falsely detected, then the burst is terminated prematurely.
Speech, because it is bursty in nature, can tolerate occasional premature terminations of the speech bursts. Human speech is very redundant and recovery of a prematurely terminated transmission is achieved upon receipt of the very next burst. Recovery for voice-band data modem transmissions and other data telephony traffic, however, is more difficult due to the fact that the burst is much longer and only one postamble is present. In this situation, a false postamble detection can cause a call to be terminated prematurely with no opportunity for recovery. Voice-band data modems, in particular, are especially sensitive to such interruptions in their transmissions.
When receiving modulated bursts, a demodulator uses an error detection threshold for detecting the postamble unique word. The error detection threshold is usually chosen to minimize the probability of missed detection for VAD-enabled transmissions.
In prior art demodulators, a single error detection threshold is used for the demodulator postamble unique word detector. Such systems, however, do not perform optimally where both speech and voice-band data/fax data are transmitted. If a voice-band data modem is present on line or VAD is disabled, frequent postamble unique word false detects can occur. As a consequence, the demodulator assumes that the burst has ended and starts searching for another burst. This causes long pauses in speech during voice calls with VAD disabled. Additionally, when using on-line voice-band data modems, false postamble unique word detects will force the call to be dropped.